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RELATIONSHIPS OF SYMMETRY 



influence may be exerted by the lie of the leaves in the bud. Its 

 leaflets are strongly asymmetric and the asymmetry appears very early, 

 long before the unfolding of the bud. This is quite opposed to 

 Hofmeister's statement. The higher edge of the leaf is covered and 

 protected by the aborting end of the leaf, the lower edge lies free between 

 the stipules, which close upon it like the shell of a mussel, and it is in 

 relation to this perhaps that we have the feebler construction of the 

 upper sides of the leaflets. Herbert Spencer has already shown that 

 in other cases it is the smaller side which is in a favourable position 

 to utilize the light inasmuch as it avoids shading. There is however 

 no case known up till now in which it has been possible to hinder or 

 to bring about asymmetry experimentally. 



The formation of the leaf of Chelidonium majus I shall briefly 

 mention here because it is very instructive (see Fig. 73). The terminal 



FIG. 74. Mimosa sensitha. Leaf. At the base of 

 each of the two chief pinnules are two 'stipelles' (reduced 

 pinnules of the second order). Natural size. 



FIG. 75. Pisutn sativuin. Portion of a shoot. 

 The stipules of the pinnate, leaf are strongly 

 asymmetric. Lehrb. 



leaflet and the lowermost lateral leaflets are nearly symmetric and the 

 latter stand out at an angle of about 45 from the midrib of the leaf. 

 The higher lateral leaflets are strongly asymmetric, the lower side- 

 lobe of each being strongly developed. In correspondence with this 

 projection the higher side of each of the adjacent leaflets exhibits an 

 indentation at the place upon which in illumination from above the 

 shade of this lobe would fall. The individual leaflets are moreover not 

 spread out in the plane of the terminal leaflet but make an angle 

 with it ; and the lowermost leaflets by their oblique position escape 

 the shadow of the higher ones. If, as in other cases, we refer this 

 construction ideologically to a relationship with light, we have to admit 

 that we do not know anything as yet about its origination. 



The leaf of Mimosa scnsitiva, represented in Fig. 74, supplies an 

 illustration of a compound leaf on which one of the leaflets of a pair 



